The present invention relates to a method and a system in a vehicle to control the input torque to a drivetrain member connected to an engine, comprising the step of determining a value of the input torque.
In vehicle technology it is known to control the torque of a transmission input shaft to reduce the wear of components on the transmission by limiting said torque. U.S. Pat. No. 5,065,319 describes a system for detecting an overtorque state at a vehicle transmission, by comparison of a determined applied torque and an allowable torque. If an overtorque state is detected a signal is outputted to decrease the torque.
A problem with known systems of torque control is that they can cause unwanted limitations in certain driving situations. For example, a heavily loaded passenger car with a trailer may need output torque levels from the drivetrain exceeding a limit level during a short period of time at low speed or stand still situations. Such a situation can arise when starting from the side of a road with a shoulder, in a steep rise.
An object of the present invention is to provide a method and a system which are adapted to reduce wear of drivetrain components and at the same time provide sufficient torque during a large number of driving situations.
This object is reached by a method and a system in a vehicle to control the input torque to a drivetrain member connected to an engine. The method comprises the steps of determining a value of the input torque, determining the rotational speed of an input shaft of the drivetrain member. A value of a first torque dependent function is calculated, based on the rotational speed, the value of the input torque and a limit value of the input torque, the value of the input torque exceeding the limit value of the input torque. If the value of the first torque dependent function exceeds a predetermined limit value of the first torque dependent function, the input torque is reduced.
Taking the rotational speed of the input shaft along with the torque into consideration when determining the input torque allowed, means that the consideration is based on parameters which together are more accurately linked to the wear of components in the drivetrain member, than what would be the case if only measured torque would be a basis for any decision to limit the torque.
Relating the torque control to the rotational speed of, e.g. The transmission makes it possible to allow high torque in situations where this is needed and it will have a relatively low impact on transmission wear. For instance, in a situation where the vehicle speed is low or zero, a large amount of torque can be needed to take the vehicle out of a difficulty, e.g. a ditch or a steep rise. Taking the low rotational speed in the vehicle drivetrain into account for overtorque control will reduce the risk of the torque being cut to prevent remedy of the situation.
The predetermined limit value of the first torque dependent function, below also referred to as accumulated torque excess function, can, in a structural fatigue perspective, be seen as a partial damage on the drivetrain member, e.g. The transmission. Preferably limit value of the first torque dependent function is taken into account in the design of the drivetrain member, and therefore the method according to the invention gives a predetermined controllable operation of the drivetrain regarding the wear of the transmission.
Preferably, if the value of the first torque dependent function does not exceed the predetermined limit value of the first torque dependent function, the procedure above is repeated to obtain a new higher value of the first torque dependent function. Thus, the torque is allowed the exceed a limit value, but as long as it does so, the closer the value of the first torque dependent function comes to its limit value, where torque is reduced.
Preferably, if the value of the input torque does not exceed the predetermined limit value of the input torque, a value of a second torque dependent function is calculated, based on the value of the input torque, the limit value of the input torque and the rotational speed, and, if the value of the second torque dependent function is lower than a predetermined minimum value of the second torque dependent function, the input torque is not allowed to exceed the predetermined limit value of the input torque. This prevents an allowed overtorque interval to be immediately followed by another overtorque interval, and provides an operational limit that will secure that the overtorque intervals are compensated for to reduce wear of the drivetrain member.